EXTERNAL ANATOMY OF INSECTS. 419 



imbibe juices, it is membranous. In form and shape it 

 varies greatly, being sometimes nearly square, at others 

 almost round ; in some insects representing a parallelo- 

 gram, in others a triangle, and in many it is oblong. In 

 some instances it is long and narrow, but more generally 

 short and wide. It is often large, but occasionally very 

 minute. In the majority it has an intire margin, but it is 

 not seldom emarginate or bilobed, or even dentate. Its 

 surface is commonly even, but it is sometimes uneven, 

 sometimes flat, at others convex, and in a few species 

 armed with a short horn or tubercle a . As to itspubescencei 

 it is often naked, but now and then fringed or clothed 

 with down or hairs, or sprinkled with bristles. It con- 

 sists in almost every instance of a single piece ; but an 

 exception to this occurs in Halictus, a little bee, in the 

 females of which it is furnished with a slender appen- 

 dage b . — The direction of the upper-lip is various. It is 

 rarely horizontal, or in the same line with the nose, often 

 vertical ; it sometimes forms an obtuse angle with the 

 anterior part of the head, and occasionally an acute one, 

 when it is more or less indexed. The use of this part is 

 ordinarily to close the mouth from above, to assist in re- 

 taining the food while undergoing the process of masti- 

 cation; but in many Hymenopterous insects its principal 

 use seems to be, to keep the tropin properly folded ; and 

 in some cases where it is inflexed, as in the leaf-cutter 

 bees (Megachile Latr.), to defend its base, while the man- 

 dibles are employed, from injury by their action c . 



a Kirby Mm. Ap. Angl. i. t. v. Apis *. b.f. 18. b. 

 " Ibid. t. ii. Melitta **. b.f 4, 5. Plate XXVI. Fig. 30. 

 c Plate XXVI. Fig. 31. Mori. Ap. Angl i. t. x. Apis **. c. 2. 5. 

 /. 13, c. 



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